For operating a gas discharge lamp, a high voltage must first be applied to the lamp in order to start the discharge process of the gas in the lamp. A continuous operating voltage must then be applied to the electrodes of the lamp. For this purpose, it is possible to use either an electrical power supply unit or a switching apparatus which can effect both the starting operation and the operating state, or else two separate voltage sources, one of which being used for starting and the other for operation. A voltage source which can be used for both states must be able to generate the high starting voltage and then be able to function continuously with high efficiency during operation.
Until, now, either superimposed-pulse ignitors or resonant circuits have been used to start discharge lamps. However, these present the following disadvantages in the case of discharge lamps having a particularly high starting voltage:
In the case of a superimposed-pulse ignitor, the operating frequency for continuous operation of the lamp has an upper limit due to the lamp's inductance. This is a substantial restriction, particularly in the case of high-pressure lamps which can be operated only in certain frequency ranges due to the acoustic resonances occurring. Superimposed-pulse ignitors are also comparatively expensive due to the windings, switch elements (for example spark gaps) and capacitors which are required.
In a series resonant circuit, a very high Q factor is required to start discharge lamps having a particularly high starting voltage by increasing the voltage, and hence costs are correspondingly high. The circuit complexity required to reliably attain the resonant frequency in such a resonant circuit is considerable. With series resonant circuits, too, the inductance limits the operating frequency for continuous operation of the lamp. It is therefore possible to use cost-effective operating equipment at high frequency to only a very restricted extent.